1. Field of the Invention
This invention relates to a method for dehydration and concentration of an aqueous solution containing an organic compound by vapor phase separation.
2. Description of Prior Art
A distillation method is generally employed as a method to dehydrate an aqueous solution containing an organic compound. For dehydrating an azeotropic mixture or a mixture of water and an organic compound having a boiling point similar to water which is hardly separated in a general distillation method, an azeotropic distillation method or an extractive distillation method is used.
For example, an ethanol-containing product produced from a biomass is processed in the following manner. The ethanol content in the product produced from a biomass is not more than 10 weight %, and hence ethanol is first concentrated to give an azeotropic composition of 95.6 weight % in a first distillation column by distillation. Subsequently, an entrainer such as benzene which produces in combination with water an azeotropic composition having a boiling point lower than ethanol is added to the concentrated product. A pure ethanol is then prepared by subjecting the resulting mixture by azeotropic distillation in a second distillation column to remove the azeotropic composition. In this process, the azeotropic distillation in the second distillation column requires a large energy to remove a slight amount of water.
As an improved method for dehydration of an organic solvent using less energy, a pervaporation method has been proposed. In this method, water vapor is selectively permeated employing a gas separation membrane by supplying an organic solution to one side of the gas separation membrane, while reducing pressure or supplying a carrier gas on another side of the membrane. However, the pervaporation method has problems in lowering of selective permeability and duration in the course of use for a long period of time, because the gas separation membrane is kept in direct contact with the organic solution and the membrane is apt to swell.
Further, a vapor-phase dehydration method has been proposed. This method employs a gas separation membrane to permeate a water vapor selectively by supplying a gaseous mixture comprising an organic compound vapor and a water vapor (which is produced by evaporation of an aqueuous solution containing an organic compound) on one side of the gas separation membrane, while keeping another side under reduced pressure or causing an inert gase as a carrier gas flowing on the surface of the membrane on another side. A method which employs a ceramic porous hollow fiber membrane is described in "Membrane" 10(5), 297 (1985, written in Japanese). The method which employs a gas permeation membrane of polyamide, cellulose or cellulose acetate is described in Japanese Patent Provisional Publication No. 60(1985)-99314.
In the method employing a ceramic porous hollow fiber membrane, the hollow fiber membrane does not swell, because the membrane is an inorganic material. However, it is disadvantageous that the ceramic porous hollow fiber is fragile to be broken easily. Further, it is difficult to form a fine hollow fiber membrane from an inorganic material. If the inorganic membrane in the form of a thick hollow fiber membrane is charged for manufacture of a module, the resulting effective membrane area capable of serving as permeation area is small and hence the membrane of an inorganic material has a problem in practical use.
Japanese Patent Provisional Publication No. 60(1985)-99314 describes a method employing a gas permeation membrane of an organic polymer of polyamide, cellulose or cellulose acetate. To dehydrate a gaseous mixture comprising an organic compound vapor and a water vapor, it is generally necessary to perform dehydration at a temperature higher than the boiling point of the organic compound-containing solution. Therefore, the gas separation membrane requires high thermal resistance and high organic solvent resistance (i.e., resistance to an organic solvent). However, the organic polymer membrane such as polyamide, cellulose or cellulose acetate, etc. is insufficient in thermal resistance and organic solvent resistance, and especially has a problem of stability of selective permeability in a long term operation. Further, the organic polymer membrane is insufficient in a permeation rate of a water vapor and selective permeability of a water vapor against a gas of an organic compound.